1. Field of the Invention
The embodiment discussed herein is related to a mask inspection apparatus and a mask inspection method by which a two-level edge structure can be determined without CAD information.
2. Description of the Related Art
As a method of measuring line widths of patterns, there is the method using a scanning electron microscope. The method using the scanning electron microscope is performed by irradiating a sample with incident electrons while scanning the surface of the sample in an electron beam scanning region by the incident electrons, acquiring, secondary electrons emitted from the sample through a scintillator, converting the quantity of the acquired electrons into luminance, and then displaying images on a display.
When managing the characteristics of a semiconductor device by using such a scanning electron microscope, a generally employed operation is to inspect whether or not a pattern is formed to have a line width within a design standard. The following procedure is used to manage the line width of a pattern. The procedure follows a sequence of displaying a predetermined region of a pattern formed on a photomask on a display, aiming an electron beam at a measurement point within the displayed region and applying it onto the measurement point, and then, acquiring a luminance distribution waveform on the basis of secondary electrons reflected from the measurement point. Subsequently, the procedure follows a sequence of determining a width of a high level portion of the luminance distribution waveform to be the line width, determining whether or not this line width falls within a tolerance range, and then, moving to the next step if the line width falls within the tolerance range while returning to the pattern-formation processing step if the line width is out of the tolerance range.
Thus, the measurement of line widths of patterns is important in the manufacturing process of semiconductor devices. Thus, there are proposed various techniques to accurately measure the line widths.
In general, an edge position of a pattern is defined by a position of the largest gradient of luminance corresponding to the quantity of secondary electrons. In contrast, Japanese Laid-open Patent Publication No. 05-296754 discloses an edge detection method which considers a position of the local minimum value of a secondary electron signal as the edge position.
As described above, in a case where a scanning electron microscope is used to measure a line width of a pattern, there is employed the method in which an edge position is defined by a position of the largest gradient of luminance or the method in which an edge position is defined by a position of the local minimum value of a secondary electron signal.
In recent years, a progress in the miniaturization of semiconductor devices brings use of photomasks having a two-level edge structure such as a phase-shift mask. For the photomasks having such a structure, the scanning electron microscopes is also used to determine whether or not a photomask is accurately created by evaluating an SEM image of the photomask.
SEM images are created on the basis of luminance data. For photomasks having a two-layer structure, the luminance in a region with a pattern is different from the luminance in a region without any pattern. Thus, a pattern structure can be detected by use of the SEM images.
However, for photomasks having the two-level edge structure, the luminance value in a region with a pattern is not necessarily different from that in a region without any pattern. Thus, it is difficult to specify the layer structure of the photomask from contrast information of SEM images.